Thermocolor ribbons have been known for some time. Generally they comprise a foil-like carrier, for example of paper, a synthetic resin or the like and a multiple-color layer which can be in the form of a synthetic resin and/or wax-bound colorant or carbon-black layer.
In the printing of a symbol upon a substrate, a printing head can apply heat and pressure to the side of the carrier opposite that provided with the transfer layer and the transfer layer thereby moved into contact with the substrate, e.g. a paper sheet adapted to receive the imprint. Under the heat and pressure applied by the printing head, which can have the form of the symbol to be printed or may be a dot matrix head progressively changing the shape of the symbol, the colored material is partly transferred to the paper and fuses thereto.
Thermal printers or printer heads for this purpose are described in German published applications DE-AS No. 2,062,495 and DE-AS No. 2,406,613, as well as in German open application DE-OS No. 3,224,445.
Utilizing such printing heads, alphanumeric characters can be applied to the paper. In these systems, moreover, the heated printing head presses the thermocolor ribbon against the paper to be inscribed. The letters of the printing fonts on the printing head are heated to a temperature of about 400.degree. C. to cause the localized melting color transfer layer to permit the locally melted portion to be transferred to the paper sheet and bonded thereto. The utilized part of the ribbon is wound on a take-up spool.
The thermocolor ribbon can have a plurality of melt colors which can be applied in succession. For example, if three base colors, blue, yellow and red are applied from respective strips of the thermocolor ribbon, a colored imprint can be generated. Utilizing this principle, it is possible to effect color printing without the development or fixing steps required in color photography. The thermal-printing head and printing mechanisms utilized at the present time are capable of operating at high speeds, for example, being capable of printing a full A4 sheet of paper in about 10 seconds, without excessive noise.
Apart from the thermocolor ribbons described, there are thermal-transfer ribbons which effect the transfer of imprints from heated symbols, not from a heated printing head, but rather from a resistance-heated foil-like carrier.
The melt-color layer which is the functional layer in the printing process, nevertheless functions in the same way. These materials are referred to as electrothermal ribbons or ETR ribbons. A corresponding thermal printing system has been described in U.S. Pat. No. 4,309,117.
Mention may also be made of the fact that thermal transfer materials are known in sheet form. Reference may be had in this regard to European patent application No. 0 260 347 published Mar. 23, 1988 and which is actuated by the priority application of the present case, to European patent application publication No. 0 152 795 of Aug. 29, 1985 and U.S. Pat. No. 4,549,824.
In general, it has been found that in the use of thermal transfer or thermocolor ribbons, the transferred symbol, i.e. the print on the substrate or paper sheet, has a shiny, highly reflective and frequently unesthetic appearance which under some circumstances can make the image difficult to perceive or the text difficult to read. The reflectivity or shininess of the transferred material is therefore a drawback which has impeded use of the thermocolor process.